U.S. patent number 5,137,026 [Application Number 07/461,089] was granted by the patent office on 1992-08-11 for personal spirometer.
This patent grant is currently assigned to Glaxo Australia Pty., Ltd.. Invention is credited to Frederick A. Ebeling, Charles K. Waterson.
United States Patent |
5,137,026 |
Waterson , et al. |
August 11, 1992 |
**Please see images for:
( Certificate of Correction ) ** |
Personal spirometer
Abstract
A self-contained portable spirometer includes a housing and an
air tube with an orifice. Within the housing there is a transducer
and microprocessor-based circuitry for generating standard exhaled
air performance measurements such as forced expiration volume and
peak expiratory flow rate, commonly referred to as FEV.sub.1 and
PFER. These measurements are displayed on a screen disposed on the
housing. Since the air tube has a non-linear response, two
amplifier stages with different amplification are coupled between
the transducer and the microprocessor. The microprocessor monitors
the flow rate through the tube and selects the signal from either
one or the other amplification stage depending on the flow through
the tube. The microprocessor also monitors the flow to determine
when no air is blown through the tube. In this latter case, the
computer shuts the power to the analog circuitry to conserve
power.
Inventors: |
Waterson; Charles K. (Chapel
Hill, NC), Ebeling; Frederick A. (Tucson, AZ) |
Assignee: |
Glaxo Australia Pty., Ltd.
(Victoria, AU)
|
Family
ID: |
23831180 |
Appl.
No.: |
07/461,089 |
Filed: |
January 4, 1990 |
Current U.S.
Class: |
600/538;
73/861.52; D24/164 |
Current CPC
Class: |
A61B
5/087 (20130101); A61B 5/09 (20130101) |
Current International
Class: |
A61B
5/09 (20060101); A61B 5/08 (20060101); A61B
5/087 (20060101); A61B 005/091 () |
Field of
Search: |
;128/725
;73/861.42,861.52,861.61 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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2941426 |
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Feb 1981 |
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DE |
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3322536 |
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Jan 1985 |
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DE |
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WO84/1704 |
|
May 1984 |
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WO |
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AU 89/261 |
|
Dec 1989 |
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WO |
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1160669 |
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Aug 1969 |
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GB |
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1351112 |
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Apr 1974 |
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GB |
|
1463814 |
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Feb 1977 |
|
GB |
|
2133157 |
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Jul 1984 |
|
GB |
|
Other References
Lecky, John H. et al "A New Disposable Spirometer" Anesthesiology
vol. 38, No. 5, Mar. 1973. .
Melia, R. J. W. et al "Suitability of a New Turbine Spirometer"
Bull. Eur. Physiopathol. Respir. vol. 21; 43-47, 1985. .
Elliott, Stanley E. "Turbulent airflow meter" J. Appl. Physiol.
vol. 42(3) pp. 456-460, 1977. .
American Thoracic Society Statement "Standardization of Spirometry"
Respiratory Care, Nov. 1987 vol. 32, No. 11 1040-1060. .
Timeter's L.A.P. Spirometry System, Aug. 15, 1987 (Product
Literature). .
Chowienczyk and Lawson, "Pocket-sized device for measuring . . . "
British Medical Jornal, vol. 285, pp. 15-17, Jul. 3, 1982. .
Saklad et al, "Pneumotachography:", Anesthesiology vol. 51, pp.
149-153; Aug. 1979. .
Heinonen, E. "Spirometers: A Field Test Evaluation", Bull. Eur.
Physiopathol. Respir. 1987 23.177-181. .
Binder, R. C. "Fluid Mechanics" 5th Edition, Prentice Hall, 1959
pp. 236, 237. .
Dalen, G. and Kjellman, B. "Valuation of Two Spirometers" Acta
Paediatr Scand--71:253-256, 1982..
|
Primary Examiner: Howell; Kyle L.
Assistant Examiner: Pontius; Kevin
Attorney, Agent or Firm: Kane, Dalsimer, Sullivan, Kurucz,
Levy, Eisele and Richard
Claims
We claim:
1. A portion spirometer comprising:
an air tube coupled to said housing and including a substantially
linear air passage with a reduced diameter orifice for generating a
turbulence in the air passage;
pressure sensing means for sensing a differential pressure across
said orifice when a person exhales through said air tube;
filter means disposed at an interface between said tube and said
pressure sensing means for protecting said pressure sensing means,
said filter means being made of a material permeable to gases and
impermeable to liquids;
electronic circuitry means disposed in said housing and coupled to
said pressure sensing means for generating electric input signals
corresponding to said differential pressure, said electronic
circuitry means including calculating means for calculating
performance signals from said electric input signals; and
display means for displaying said performance signals.
2. The spirometer of claim 1 wherein said pressure sensing means
includes access holes disposed in said air tube and spaced from
said orifice, and transducer means coupled to said access holes for
generating a transducer output corresponding to said differential
pressure.
3. The spirometer of claim 1 further comprising power supply means
disposed in said housing for supplying said circuitry means with
electrical power, said circuitry means deactivating said power
supply means when said spirometer is in an idle mode.
4. The spirometer of claim 3 wherein said electronic circuitry
means includes analog circuit means and digital circuit means, and
wherein said power supply means provides power to said analog
circuit means when said spirometer is not in said idle mode.
5. The spirometer of claim 3 wherein said power supply means
includes a battery to make said spirometer self-contained.
6. The spirometer of claim 1 further comprising voltage offset
compensation means for offsetting voltage offsets in said circuitry
means.
7. The spirometer of claim 6 wherein said voltage offset
compensation means includes digital-to-analog convertor means for
receiving an error signal from said means corresponding to said
voltage offset and said converter means generating a convertor
output.
8. The portable spirometer of claim 1 wherein said electronic
circuitry means includes first amplifier means for amplifying said
electric input signals by a first factor to generate a first
amplified output, second amplifier means for amplifying said
electric input signals by a second factor to generate a second
amplified output, and microprocessor means being coupled to first
and second amplified output, said microprocessor means being
programmed to select one of said one of said first and second
amplified outputs to calculate said performance signals.
9. The portable spirometer of claim 8 wherein said electronic
circuitry means further includes analog-to-digital converter means
for converting said first and second amplified output for said
microprocessor means.
10. The portable spirometer of claim 8 wherein said air passage has
a non-linear response to air flow, and wherein said microprocessor
means includes select means which selects said first amplified
output in response to a first air flow below a threshold level, and
wherein said select means selects said second amplified output in
response to a high air flow above said threshold level.
11. The portable spirometer of claim 8 wherein said microprocessor
means includes sampling means which samples the air flow through
said air passage and which calculates a running average signal.
12. The spirometer of claim 11 wherein said microprocessor means
includes monitoring means which monitors said average signal to
discriminate a test from noise.
Description
BACKGROUND OF THE INVENTION
1. Field of Invention
This invention pertains to an apparatus for automatic measure of
the volume and flow rate of air exhaled by a person, and more
particularly, to a personal spirometer small enough so that it can
be carried unobtrusively in a pocket so that person can use it
easily with maximum convenience and minimum embarrassment.
2. Description of the Prior Art
Spirometers are devices used to measure the volume and flow rate of
air exhaled by a person. These measurements are important of
general physiological studies and for diagnostic analysis of
particular patients. For example, the effects of various medicines
used to treat patients with pulmonary or asthmatic problems can be
best analyzed by monitoring the volume and flow rate of air exhaled
at regular intervals before and after the administration of
medication.
In general, spirometers make their measurement by one of two means.
One type collects the exhaled volume from the subject into a
bellows or other container, the displacement of which corresponds
to the volume of exhaled air. These devices are by their nature
large to allow sufficient air collection volume and hence are not
easily made portable. A second type measures the rate of air flow
through a flow measurement device. Exhaled volume is derived by
integration of the air flow rate over some period of time.
Until the present invention, spirometers were rather bulky and
expensive devices found mostly in clinics and laboratories. Their
operation required a trained technician. Furthermore, most such
devices were complicated so that they could not be made small
enough to be carried in a pocket. For example, there are several
devices available in the market known as pneumotachs, such as the
Fleisch Pneumotach. These devices depend on a laminar air flow past
a resistance element. Such devices need additional means for
insuring that the flow remains laminar even at high air velocities.
Therefore, these type of devices are inherently complex and
relatively large. Furthermore the resistance element frequently
includes a screen disposed directly in the air path. This screen
intercepts impurities which clog the screen and change the response
of the device and in addition are unsanitary. The pneumotach also
includes pressure measurement ports which frequently become
occluded from moisture or impurities, thus adversely altering the
accuracy of measurement.
OBJECTIVES AND SUMMARY OF THE INVENTION
An objective of the present invention is to provide an apparatus
which may be used for an accurate and instantaneous measurement of
exhaled air.
A further objective is to provide a self-contained apparatus which
can be made small enough to fit in a person's pocket.
Another objective is to provide an apparatus which is simple enough
to be used by the subject and is reliable without special care from
the user.
Yet another objective is to provide an apparatus which can be
adapted for data recording.
Other objectives and advantages of the invention shall become
apparent from the following description of the invention. Briefly,
an apparatus constructed in accordance with this invention includes
a housing with an air tube in which turbulent flow is induced when
a person blows air therethrough. The air tube is adapted for
measurement of the flow rate of air exhaled therethrough. A
pressure transducer within the housing senses a pressure
differential created by said exhaled air and produces electrical
signals indicative of said pressure differential. Signal processing
circuitry within the housing processes these electrical signals to
calculate the volume and rate of exhaled air and display the same
on a display window. Preferably, the spirometer displays two
parameters known as FEV; (the volume of air exhaled in one second
in liters) and PEFR (peak expiratory flow rate in liters per
second). Optionally the results of several measurements may be
recorded in a memory for later down-loading to a data processing
system.
An adaptive start algorithm is used to detect a true test, using a
statistical approach rather than a preset threshold level. More
particularly, the device calculates the average and the variance of
a window formed of four consecutive samples and flow rate and
volume calculations are started only if a progressive increase in
the measurements is detected. This approach is found to provide
good sensitivity and, at the same time, it is relatively immune to
noise.
Furthermore, the spirometer takes advantage of the non-linear
characteristics of the air tube to increase sensitivity without
expensive high resolution A/D converters. Separate look-up tables
for high and low flow rates are used to convert pressure
differential samples into actual flow rates.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a side view of a personal spirometer constructed in
accordance with the invention;
FIG. 2 shows a front view of the spirometer of FIG. 1;
FIG. 3 shows a cross-sectional view of the air tube of FIGS. 1 and
3;
FIG. 4 shows an end view of the air tube of FIG. 3;
FIG. 5 shows somewhat schematic block diagram for the elements of
the spirometer constructed in accordance with this invention;
FIGS. 6A and 6B show an elementary wiring diagram of a preferred
embodiment of the invention; and
FIGS. 7A, 7B and 7C show flow charts for the operation of the
microprocessor for the spirometer.
DETAILED DESCRIPTION OF THE INVENTION
Referring now to the drawings, a personal spirometer 10 constructed
in accordance with this invention includes a housing 12 with a
generally square section 14 and an air tube 16 disposed on one side
of the section 14. The spirometer is sized and shaped so that it
can fit in a pocket. Furthermore, the spirometer is shaped and
sized so that it can be held comfortably in one hand while air is
exhaled through it as described more fully below. The section 14
has a flat surface 18. A control panel 20 is imbedded in surface 18
and it includes a start button 22' and an LCD display screen 24.
Air tube 16 has an annular mouth piece 26 at one end sized to fit
in a person's mouth. A cylindrical hole 28 passes through the air
tube 16. Hole 28 has a substantially constant diameter except at an
annular wall 30. This annular wall 30 forms a sharp-edged orifice
within hole 28. Two small openings 32, 34 are spaced on either side
of the wall 30 and extend into section 14 for measuring the
differential pressure within the air tube due to a flow of exhaled
air.
As shown more clearly in FIG. 4 each of the openings 32, 34 is
provided with a plug 36, 36' (only one orifice being shown in FIG.
4). This plug holds a filter 38 at the interface with hole 28. The
filter 38 may be made of a porous material which is permeable to
air but impermeable to liquids. In this manner, saliva or other
materials from the exhaled air of a person will be limited to the
tube and will not contaminate the remainder of the spirometer 10.
Furthermore since filter 38 is impermeable to water the spirometer
may be immersed in or sprayed with water for cleaning and sanitary
purposes. Filter 38 maybe made for example of a hydrophobic filter
media such as a 1/16" thick hydrophobic polyethylene with a 10
micron pore size, and about 40% porosity.
As shown more clearly in FIG. 5, the plugs are connected by two
tubes 40, 42 to a differential pressure transducer 44, which may be
for example a MPX 2010D made by Motorola.
The transducer 44 generates an electrical signal on a pair of
output wires 46, which signal is proportional to the differential
pressure between tubes 40, 42. This signal is amplified by a
differential amplifier stage 48 and fed into an analog-to-digital
converter 50 which converts the amplifier output into digital
signals. The converter output is fed to a microprocessor 52. The
microprocessor 52 uses an algorithm stored in a ROM 54 to perform
several calculations on the signal from converter 50, and to
display the results (i.e. volume and rate of flow) on display 24.
Switch 22 activated by button 22' initiates the operation of
spirometer 10 through microprocessor 52. The results obtained
during each measurement may be stored in a RAM 56 for future
reference. An input/output port 58 may also be provided to allow
for changing the programming of the microprocessor. Furthermore the
microprocessor may be programmed so that on command it can
down-load the results accumulated in RAM 56 through port 58 to a
printer or a desk-top computer.
A preferred diagram for implementing the circuit shown in FIG. 5 is
shown in FIGS. 6A and 6B. It should be understood that the various
circuit elements (such as resistance and capacitance values) are
shown in the Figures merely for illustrative purposes and do not
limit the scope of this invention in any fashion.
In the embodiment of FIGS. 6A and 6B, differential air pressure in
the air tube is sensed by the pressure transducer 44 schematically
shown as a resistive bridge. The output of the transducer is
processed by an amplifier analog circuit 48 consisting of
amplifiers 60, 62, 64 and 66 which may be, for example, Motorola MC
34074 op amps. These amplifiers are used for a relatively high air
flow. For low air flows a further amplifier 68 is also used. The
outputs of these amplifiers 66, 68 are fed to a multiple channel
A/D converter 50, which may be for example a 68HC68A2 manufactured
by RCA Harris. The A/D converter 50 feeds its output to
microprocessor 52 which may be for example a MC68HC804C4 made by
Motorola. After performing the necessary calculations, the
microprocessor 52 displays the results on the LCD screen 24.
(Display screen 24 may also include LCD display drivers not shown
in the Figures for the sake of convenience).
As shown in FIG. 6A, the circuit also includes a power supply 70
which provides the required power to the various circuit elements
from a battery 72. The operation of the power supply is also
controlled by wires W2, W3 by the microprocessor 52. More
particularly, the analog section consisting of the amplifiers, the
transducer and the digital-to-analog converter is turned on last
(when measurements are started) and turned off first (when the
measurements are completed) to conserve power. The power to the
display screen is independently controlled. Preferably, the display
is on whenever the microprocessor is on.
The spirometer 10 further includes a beeper 80 controlled by the
microprocessor for generating audible signals for the user.
A further feature of the invention is a automatic offset
compensation circuit consisting of plurality of resistors 74 and an
amplifier 76. The resistors 74 are coupled to microprocessor 52 by
a plurality of lines 78. This offset compensation circuit operates
as follows. During the initialization of the spirometer (described
more fully below), the microprocessor checks the output of the
pressure transducer to insure that it essentially corresponds to no
air flow. If the transducer output is non-zero (due for example to
a temperature drift, a variation in the output of the power supply
70, the offset voltages of amplifiers 62, 64, 66, 68 and so on) the
microprocessor 52 sends a compensating signal through lines 78 to
resistors 74. Resistors 74 and amplifier 76 cooperate in effect to
form a digital-to-analog converter used by the microprocessor 52 to
produce a DC offset. This DC offset is added by amplifier 62 to the
output of transducer 44. During the initialization period when no
air is blown through the air tube, the microprocessor sequentially
changes the signals on lines 78 until the offset signal from
amplifier 76 compensates for the error signal from transducer
44.
OPERATION
The device requires no user adjustment or calibration. To make a
measurement, the user pushes the START button 22'. This turns the
unit on and initiates a self-test routine. During this self test,
all segments on the liquid-crystal display (LCD) are turned on to
allow the user to confirm proper operation of the unit. Upon
completion of self-test (approximately 5 seconds), the display is
blanked except for a READY annunciator: the unit beeps by activity
beeper 80 and is now ready for a measurement. The user inhales as
much as he can, places his lips around the mouthpiece 26, and blows
as hard as possible. The device senses the start of exhalation,
measures flow for one second, then displays performance signals
such as the forced expiration volume and maximum rate of air flow
for the person (commonly known as FEV; and PEFR respectively)
measurements on display screen 24.
The parameter FEV; and the criteria for measuring this parameter is
described in the Official Statement of American Thoracic Society,
Medical Section of the American Lung Association --Standardization
of Spirometry--1987 Update found in Respiratory Care, November '87,
Vol. 32, No. 11, pgs. 1039-1060. The parameter PEFR is identical to
the FEF.sub.max parameter in the same Statement.
The display will persist for 45 seconds, and then the unit will
turn itself off, unless the START button 22' is pushed to initiate
another measurement cycle. If no breath is detected within 15
seconds of the READY signal, the unit beeps twice and shuts itself
off.
PRINCIPLE OF MEASUREMENT
The spirometer 10 determines the flow rate of air by measuring the
differences in pressure developed across a restricting orifice.
This pressure difference is related to the flow rate by a
well-known equation based on Bernoulli's equation for
non-compressible flow. (See for instance Binder, R. C., Fluid
Mechanics, 5th Edition, Prentice Hall Inc., Englewood Cliffs, N.J.,
pgs. 236-237.) In the case of the sharp-edged orifice used in this
device, the flow rate is equal to a coefficient (found empirically)
multiplied by the square-root of the pressure difference measured
between a point upstream of the orifice and a point downstream of
the orifice. The value of the coefficient is predominantly
determined by the physical design of the device, including the
ratio of the area of the flow tube to the area of the orifice, the
size of the orifice, and the location of the pressure measurement
ports. Ideally, if these physical parameters were held constant,
the pressure difference would be dependent only upon the flow rate
and density of the fluid being measured. However, there is also
some influence of Reynolds Number upon the value of the
coefficient, which introduces an error if the coefficient is
treated as a constant over a large range of flows.
The pressure difference across the orifice is a function of the
square of the flow rate. Therefore, an orifice size must be chosen
that does not offer excessive back-pressure to the highest flows to
be measured, yet has an adequate, measurable pressure difference at
low flow rates.
THEORY OF OPERATION
The air tube 16 contains the sharp-edged orifice (defined by wall
32) that provides a pressure difference which is approximately
proportional to the flow rate squared. Preferably, the diameter is
about 5/8" and tube 16 has a diameter of about 7/8". This size
represents a reasonable compromise between back-pressure at higher
flows and at low flows. The outside diameter of the tube 16 is
approximately 1", and the length is approximately 3.5". Again, the
dimensions represent a compromise; an attempt has been made to keep
the overall size small enough to fit a pocket or handbag, yet large
enough so that an extraneous mouthpiece is unnecessary. However, a
tapered profile is provided on the inlet end of the tube so that a
disposable mouthpiece may be added (26' in FIG. 1) if desired. The
pressure ports are covered with a disk of hydrophobic filter
material (as described above) inset flush with the floor of the
tube. This material allows air and water vapor to pass freely, but
blocks dirt and liquid. It is made of a 1/16" thick rigid plastic
and is not easily damaged, allowing the interior of the tube to be
cleaned with gently running water or wiped with a soft, lint-free
cloth.
The pressure is transmitted via the 1/16" i.d. pipes 40, 42 to the
solid-state, piezo-resistive, differential pressure transducer 44.
This transducer is provided with a reduced amount of silicon
isolation gel coating its diaphragm as compared with the standard
transducers used for other measurements. This coating improves the
transient response and reduces the sensitivity of the transducer to
the position and motion of the spirometer. The differential
pressure transducer provides an output signal proportional to the
pressure difference between the two openings 32, 34.
The signal from the transducer is amplified and filtered by the
4-stage analog amplifier circuit shown in FIGS. 6A and 6B. Two
outputs are produced by this amplifier circuit. The first circuit
generated by amplifier 66 has a total gain of 808. The second
circuit generated by amplifier 68 has a total gain of 3,232.
The offset voltage of this circuit is adjusted to 300 +/-50 mV at
the first output as described above.
Two filter stages (including amplifiers 64, 66) are included in the
analog amplifier circuitry providing a low-pass transfer function
with a cut-off frequency of 10 hz. The first and second outputs
from the analog circuitry are fed to two channels marked CH1, CH3
of the 10-bit A/D converter 50. A signal proportional to th voltage
of battery 72 is fed to channel CH2 on the A/D converter 50.
The microprocessor controls all aspects of device function. It is
able to independently control power to the display, the pressure
transducer and analog circuitry, and itself. Timing pulses are
provided by a 3.59 MHz crystal 53. The microprocessor receives the
digital values representing pressure, does all necessary
calculations, and generates the codes for the liquid-crystal
display circuitry.
The liquid-crystal display shows the measured values for FEV.sub.1
and Peak Expiratory Flow Rate (PEFR). It also includes a BATTERY
annunciator to indicate when the battery needs replacement and a
READY annunciator to indicate when the device is ready to make a
measurement.
MEASUREMENT SEQUENCE
The sequence of operation for the spirometer is now described in
conjunction with the flow charts of FIGS. 7A, 7B and 7C. Details of
the operations are found in the program listing attached hereto.
When the start switch is depressed, the microprocessor 52 is reset
and loads its program, which is stored in its Read-Only Memory
(ROM) 54. It begins by running a self-test and initialization
routine step S1 which checks for internal consistency. It also
measures the voltage of battery 72 through input channel CH2 of the
A/D converter 52. If the voltage is below a lower limit such that
an accurate measurement cannot be made, the microprocessor will not
continue. If the voltage is low, but does not exceed this
operational limit, the BATTERY annunciator on the LCD is turned on
and will not extinguish until the unit turns itself off. If the
internal checkout is completed without error, the microprocessor
then turns on all segments of the display to allow the user to see
if any segments are non-functional. Next, it starts sampling the
input on channel CH1 of the A/D converter so as to decide which
bits of the digital-to-analog converter (I/O lines 78) to turn on
or off and to adjust the offset voltage at channel CH1 to 300+/-50
mV. This sequence takes about 2 seconds.
At the end of offset adjustment, the display is blanked (except for
the battery annunciator, if voltage is low) and then analog
circuitry is allowed to stabilize. The microprocessor in step S2
begins to sample both CH1 and CH3 inputs of the A/D converter
signals at a rate of about 100 Hz and fills its history array as
described below.
The subroutines for step S2 are shown in FIGS. 7B and 7C. The unit
continues to sample both the CH1 and CH4 channels at 100 Hz each.
It continuously calculates the average and variance of the past
four measurements X1, X2, X3 and X4 from the CH3 channel (Steps
S21, 22 and 23). It then compares the next sample (X5) to this
average and variance in Steps S24, S26. If the current value is
higher than the average by more than 2 standard deviations, the
unit branches to a start detection routine (Step 27). If the value
is more than 2 volts higher than the average, the microprocessor 52
switches over to the CH1 channel (Step 28) and scales the reading
(Step 29). If the current value is lower than the average or is
less than 2 standard deviations higher than the average, the
current value becomes the new 4th sample in the average and
variance calculation and the loop continues (Steps 22, 30, 39).
This loop will continue for a maximum of 15 seconds. If no start is
detected (as described below) in this time, the unit will beep
twice and turn itself off in step S8.
If the current sample value is high enough to cause branching to
the start detection routine, the old average and variance are
saved. In Steps 31, 32 the next sample is now checked to see if it
is also above a threshold level 128 (Step 32) or the 2 standard
deviation threshold (using either the CH3 or CH1 channel, depending
on how large the input on channel CH3 is), and also to see if it is
higher than the previous sample (Step 33). If both of these
conditions are met, a third sample is obtained and checked in the
same way (Step 34). (It must be larger than second sample.) If
either the second or third sample fails either test, the average
and variance are updated using the new sample values and the
program returns to the loop above (Steps 35, 36, 37, 38, 39). If
three samples in a row are larger than the threshold value 128 or
the average plus 2 standard deviations, and each is larger than the
one preceding it, then start is detected (Steps 40, 41, 42, 43).
The saved average is stored as the offset to be subtracted from all
samples and the 3 samples are converted to flow values and summed
as the first three volume increments (Step 41). The actual
conversion from the measurement pressure differential samples to
volumes is accomplished by using two look-up tables stored in ROM
54. One look-up table correlates samples from the low flow channel
CH3 (i.e. 0-2 volts) while the second look-up table correlates the
samples from the high flow rate channel CH1 (i.e. 0-4 volts). The
values on these look-up tables are determined empirically. As
previously mentioned, the orifice used to measure flow rate has a
non-linear response, i.e. the pressure differential across the
openings 32, 34 due to turbulent flow is non-linear. The present
spirometer takes advantage of this non-lineality by separating the
pressure differential samples into two ranges based on the flow
rate, and then using a look-up table for each. By switching gains,
the microprocessor has expanded resolution at low flows. This would
be analogous to an equivalent linear range of 16 volts at 10 bits
of resolution, or 4 volts at 12 bits of resolution. This feature is
made possible by the non-linear characteristic of the orifice. To
conserve microprocessor memory space, rather than store a value for
every possible A/D code, the microprocessor extrapolates between
the two closest stored values. Enough samples are stored to keep
the extrapolation error small. In this manner, a
less-discriminating (having lower resolution) A/D converter can be
used without sacrificing accuracy and sensitivity.
Once start is detected and integration of flow values has begun,
the unit begins to look for the maximum slope of the volume curve
(step S3). If the slope determined by several consecutive samples
is low or negative, the whole measurement is reported as a false
start. The maximum positive slope is determined over seven flow
samples and, when found, is back extrapolated to determine the
start of the first second timing for FEV; determination in
accordance with the standards set by the American Thoracic Society
identified above (step S4). Meantime, each 10 msec. the CH1 and CH3
channels are sampled (step S5). If the CH3 output is less than 2
volts above the offset, it is converted to flow and summed. If the
CH3 input is more than 2 volts, the input from channel CH1 is used
instead. The unit also looks for, and stores the highest flow
sample (step 6).
When the FEV.sub.1 timing has been determined, flow samples have
been collected and integrated as volume, and the peak flow value
has been stored, the unit displays the measured FEV.sub.1 and PEFR
(step S7) and it turns off the analog circuitry to save battery
life. The display is maintained for 45 seconds (or until the START
button 22' is pushed to initiate another measurement cycle). After
45 seconds, the microprocessor powers down (step S8) to an idle
mode.
The operation and circuitry described above and in FIGS. 6A, 6B and
7 pertain to a basic spirometer. For more advanced modes additional
features may be incorporated mostly by modifying the programming of
microprocessor 52. For example, before the unit goes into the idle
mode, each measurement may be stored into RAM 54 with a time stamp
and/or date stamp indicating the time and day on which the
measurements were made. The measurements are then recalled and
reviewed on the display screen sequentially by activating switch
22. As an incentive, the instantaneous flow measurements could be
displayed as the person blows through air tube 26, and when certain
mile stones are reached, the beeper could be sounded.
Obviously numerous other modifications can be made to the invention
without departing from its scope as defined in the appended claims.
##SPC1##
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